This invention relates broadly to a refrigeration machine, and more particularly to a high tonnage, low pressure refrigeration machine. Still more particularly, this invention relates to a refrigerant flow control device particularly equipped to pass liquid and gaseous refrigerant under normal and low load conditions, and to precisely control the flow of refrigerant by a precisely sized annular port.
In high tonnage refrigeration machines, a compressor is arranged to extract gaseous refrigerant from an evaporator, compress the refrigerant, and pump it to a condenser where it is cooled and condensed. In communication with the condenser, there is normally provided a receiver for collecting liquid refrigerant formed in the condenser prior to its passage to the evaporator. The receiver usually contains refrigerant both in the liquid and gaseous phases. The level of the liquid refrigerant collected in the receiver is a function of the pressure difference between the high pressure side and the low pressure side of the machine and the load on the machine, it being understood that the condenser and the receiver are disposed in the high pressure side and the evaporator in the low pressure side of the machine. Refrigerant flow from the receiver to the evaporator is regulated by valve mechanisms to pass an amount related to the load on the machine and automatic controls such as suction guide vanes are provided for the purpose of maintaining flow rates corresponding to machine loading. The valve mechanisms disclosed by the prior art for passing the proper amount of refrigerant from the receiver to the evaporator are adapted to meet two operating conditions of a high tonnage refrigeration machine: normal operating conditions and low loads operating conditions.
For normal operating conditions there is provided a float valve mechanism in the receiver which passes the proper amount of refrigerant as determined by the level of the liquid therein, which, in turn is determined by the loading on the machine. The evaporator contains a multiplicity of tubes usually referred to as tube bundles through which the medium to be cooled by the machine is passed in heat transfer relation with the refrigerant and the evaporator. Under these conditions, the heat supplied to the refrigerant from the medium being cooled is sufficient to cause the liquid refrigerant in the evaporator to boil vigorously. The boiling refrigerant thus wets all the tubes in the evaporator, providing optimum heat transfer between the medium and the refrigerant.
At low loads, on the other hand, the refrigerant in the receiver is primarily in the gaseous state with the liquid level being below the level necessary to actuate the float valve mechanism to the open position. Moreover, at low loads, a relatively small quantity of heat is given off by the medium and consequently the refrigerant does not boil vigorously enough to wet all the tubes, particularly those in the upper rows of the bundle, causing the efficiency of the evaporator to decrease. Also, under extended periods of low load or extremely low loads, the refrigeration machine may become unstable. To overcome these practical difficulties, the prior art discloses various electrical, pneumatic, and mechanical devices which actuate the float valve in response to an operating characteristic of the refrigeration machine, regardless of the liquid level in the receiver. The gaseous and liquid refrigerant passed by the remote actuation of these devices maintains high evaporator efficiency at low loads by agitating the liquid refrigerant in the evaporator. It also stabilizes the refrigeration machine by maintaining proper pressure differential between the high and low pressure sides.
It is seen then that the refrigerant flow control devices disclosed by the prior art have two separate and distinct flow control mechanisms, one which will operate only under normal load conditions, and the other which will operate only under low load conditions. Such devices have the disadvantage of numerous moving parts, which may influence their reliability and the size and shape of the receiver. Moreover, such low load flow control devices are usually electrically, mechanically, and/or pneumatically complex, requiring a high degree of skill in calibrating, maintaining, and repairing them.